Thursday, 21 March 2013

The universe after Planck

Here's the 2013 rendition of the cosmological Mona Lisa,
or, in our jargon, the multipole expansion of the Cosmic Microwave Background (CMB) power spectrum. Compared to previous experiments, what distinguishes Planck is that it measures the power spectrum all the way from the largest angular scales down to less than 0.1 degrees. Before, to cover the entire range, one had to combine several different CMB experiments: WMAP, SPT, ACT, which was more vulnerable to unknown systematic errors (indeed, the results from SPT and ACT were not completely consistent). Thanks to Planck, the errors are reduced, especially at large multipoles, and the confidence in the results is strengthened.

For the general public, the most palatable piece of information is about the composition of the Universe. The dough is made of 69.2±1.0% dark energy, 25.8±0.4% dark matter, and 4.82±0.05% baryons, after combining Planck with other cosmological measurements. As you can see, the errors are of order 1 in 100, which represents a factor of 2 increase in precision compared to WMAP-9. The central values have shifted a bit: there's additional 2% of dark matter at the expense of dark energy, but the change is consistent within 2 sigma with previously quoted errors.

From a particle physicist's point of view the single most interesting observable from Planck is the notorious Neff. This observable measures the effective number of degrees of freedom with sub-eV mass that coexisted with the photons in the plasma at the time when the CMB was formed (see e.g. my older post for more explanations). The standard model predicts Neff≈3, corresponding to the 3 active neutrinos. Some models beyond the standard model featuring sterile neutrinos, dark photons, or axions could lead to Neff > 3, not necessarily an integer. For a long time various experimental groups have claimed Neff much larger than 3, but with an error too large to blow the trumpets. Planck was supposed to sweep the floor and it did. They find Neff=3.3±0.5, that is no hint of anything interesting going on. The gurgling sound you hear behind the wall is probably your colleague working on sterile neutrinos committing a ritual suicide.

Another number of interest for particle theorists is the sum of neutrino masses. Recall that oscillation experiments tell us only about the mass differences, whereas the absolute neutrino mass scale is still unknown. Neutrino masses larger than 0.1 eV would produce an observable imprint into the CMB. In fact, the SPT experiment recently made a claim that the sum of neutrino masses is 0.32±0.11 eV, a 3 sigma evidence for a non-zero value. That would be huge, if confirmed. Well, no such luck:
Planck sees no hint of neutrino masses and puts the 95% CL limit at 0.23 eV. (Check out the comment section for a more informed statement).

Literally, the most valuable Planck result is the measurement of the spectral index ns, as it may tip the scale for the Nobel committee to finally hand out the prize for inflation. Simplest models of inflation (e.g., a scalar field φ with a φ^n potential slowly changing it vacuum expectation value) predicts the spectrum of primordial density fluctuations that is adiabatic (the same in all components) and Gaussian (full information is contained in the 2-point correlation function). Much as previous CMB experiments, Planck does not see any departures from that hypothesis. A more quantitative prediction of simple inflationary models is that the primordial spectrum of fluctuations is almost but not exactly scale-invariant. More precisely, the spectrum is of the form P∼(k/k0)^(ns-1), with ns close to but typically slightly smaller than 1, the size of ns-1 being dependent on how quickly (i.e. how slowly) the inflaton field rolls down its potential. The previous result from WMAP-9 ns=0.972±0.013 (ns=0.9608±0.0080 after combining with other cosmological observables) was already a strong hint of a red-tilted spectrum. The Planck result ns=0.9603±0.0073 (ns=0.9608±0.0054 after combination) pushes the departure of ns-1 from zero past the magic 5 sigma significance. This number can of course also be fitted in more complicated models or in alternatives to inflation, but it is nevertheless a strong support for the most trivial version of inflation.

I was a bit surprised by how much emphasis in today's press conferences was put on the small glitches at low multipoles. It seems that Planck people are also a bit frustrated by the fact that their results are nothing but a triumphant confirmation of old paradigms. Even at the LHC nobody would make a big deal of a 2.5 sigma anomaly, and in the present case we're in the area of astrophysics where error bars are treated more loosely ;-) Moreover, according to Planck, the quadrupole mode in the fluctuation spectrum is aligned with the ecliptic plane, which suggests some unknown background or pesky systematics at large angular scales. Of course, many a theorist will come up with a beautiful explanation of the low multipole anomaly. But not because it's convincing, but because there's nothing else to ponder on...

In summary, the cosmological results from Planck are really impressive. We're looking into a pretty wide range of complex physical phenomena occurring billions of years ago. And, at the end of the day, we're getting a perfect description with a fairly simple model. If this is not a moment to exclaim "science works bitches", nothing is. Particle physicists, however, can find little inspiration in the Planck results. For us, what Planck has observed is by no means an almost perfect universe... it's the perfectly boring universe.

Mind that this post is an outsider's perspectivefrom theangle of particle physics. For a better insight into cosmological aspects of the Planck resultssee for example here. Note that Planck dumped 28 full-grown papers today, so browsing through it will take some time, andthere may be some hidden treasures at the bottom of the chest...

36 comments:

SPT only made a claim of 3-sigma evidence for a non-zero sum of neutrino masses when they combined their CMB power spectrum measurements with information from the abundance of galaxy clusters that they've detected with the SZ effect.

If you take a look at paper 20 in Planck's release today you'll actually find *exactly* the same claim! (I've no idea why Planck chose to focus on those not-so-significant large-scale anomalies you mention in your post, instead of some of their actually interesting results, such as this)

Of course, for both SPT and Planck these results rely heavily on our understanding of scaling relations between the SZ signal significance and mass. So, it could just be that the masses of SZ detected clusters are being systematically under-estimated.

But, then again, they might not (and there is no reason to think that they are). These clusters could be showing the effects of non-zero neutrino masses...

An interesting result is the continuing tension in the Hubble rate, which is systematically lower in many estimates than direct measurements of supernovae. Planck has ~68 km/s/Mpc compared to ~73. I can't imagine there being any interesting particle physics behind this, though. This low value is what causes Planck to see less dark energy, more dark matter, and consequently higher matter fluctuations. The parameter \sigma_8, the variance in matter fluctuations on 8 Mpc scales, comes out higher than some other estimates because of this.

One possible explanation for the newly verified dipole anisotropy in the CMB is that the structure of the cosmos has a fractal geometry and nature's hierarchy extends far beyond the observable universe.

Unlike the radical idea of a multiverse of 10^500 different universes with random properties, the discrete fractal paradigm proposes one unified physics for the entire cosmos. It is a new paradigm that is based on enlarging the symmetry properties of nature, rather than invoking ad hoc and thoroughly untestable speculations.

The number I got from reading the papers for Neff was 3.3 +/- 0.27, which considering the 3 active neutrinos is 3.046 and that four active neutrinos is seomthing like 4.05ish puts the result about 0.8 sigma from 3 and about 3 sigma from 4. The size of the MOE greatly impacts the relative probabilities.

The paper also notes that the sum of neutrino masses can't be less than 0.06 eV or so, with a normal hierarchy and not less than 0.1 eV with an inverted one. Also, while there is a 95% CI at 0.24 eV or less, if you make the call that for non-Planck reasons there are really precisely three active neutrinos and look at the bounds at Neff=3.046 you get a bound more like 0.18 eV. The "weak" best fit is 0.06 eV, very close to the minimum allowed by other physics.

With three neutrino types in a normal hierarchy and 0.24 eV total, you get a 0.06 eV electron neutrino, with 0.18 eV total you get a 0.04 eV electron neutrino. These aren't very stringent bounds, really. If I had to bet, I'd put my money on a normal hierarchy and an electron neutrino of equal to or less than 0.004 eV on the theory that the electron neutrino mass ought to be meaningfully less than the difference in mass between the lightest neutrino mass and the second lightest neutrino mass. On the other hand, we now know the absolute neutrino masses with a precision comparable to the precision with which we know the up and down quark masses.

Sterile neutrino proponents, however, should put down the guns. Any sterile neutrino that is even 10s or 100s of eV, let alone keV, MeV or GeV scale, wouldn't show up in Neff. All of the interesting sterile neutrinos are heavy dark matter candidates and hot dark matter (which an Neff included sterile neutrinos would be) was pretty soundly ruled out almost immediately.

But, the folks at LSDN and MiniBooNE who see a reactor anomaly that could be a sterile neutrino species at about 1.3 eV (not confirmed by other measurements) should brace for disappointment, because Planck caps potential fourth sterile neutrino mass in the event that there is one, at about 0.5 +/- 0.1 eV, a possiblity that makes reactor anomaly and Planck data inconsistent at more than two sigma.

The paper notes in passing that a fractional value of more than 3.046 Neff but less than 4 could be consistent with an additional type of radiation particle, as opposed to another relativistic fermion.

Yes, sure, it is still possible to have additional light species, light sterile neutrinos in particular. The big difference is that before Planck one could argue that sort of thing is suggested by the data. Now it's merely not excluded...

Doddy, it is actually the other way around on sigma_8: Planck lensing, measured at 20-25 sigma, constraints matter density times sigma_8 and since matter density is higher than before sigma_8 is lower than it would have otherwise been given that measurement.

The fact that even with the higher mass density sigma_8 is higher than before is just another way of saying that their lensing amplitude is very very high compared to anything else out there, including their own cluster abundance constraint. It will be interesting to see if this will survive the scrutiny of time.

Despite the continuing negative, frustrated hype about the crisis in particle physics by many people – including you, Jester – the life in other disciplines including cosmology is much more boring and unusually revolutionary erase fade away more quickly than they do in particle physics.

life in cosmology is not boring at all in those days ! we have plenty of results and hints for new physics! Maybe you don't see this if you don't work in the field. I would never move to particle physics now :-D

I think the fact that Planck would pay more attention to the "anomalies" than WMAP could always have been predicted, given the names in the author list.

For instance, off the top of my head: Martinez-Gonzalez, Vielva, McEwen, Cruz etc. from the original papers that pointed out the Cold Spot, Eriksen, Hansen, Górski etc. from the hemispherical asymmetry papers, Räth and others from phase correlations, Kim and Naselsky from parity asymmetry and so on. (Apologies to any authors reading this whose names I may have forgotten.)

Hi, concerning the bound on Sum(m_nu), giving a closer look to the XVI Planck paper (sect.6.3.1), I got the impression that the 0.23eV result is far from being a "conservative bound" (and seems to be largely driven by BAO, not by CMB). Actually I don't understand why the disregard the lensing, that favor non-zero values.

Anyway, in their conclusion they prefer to quote 0.66eV from CMB data only.

What you say on Neff isn't really correct and the situation has not actually changed that much. The CMB only measurements give a value about 1 sigma larger than 3 (this is the value you quote) - this was also true before. If this is combined with the astrophysically measured value of H0 (the Riess value of around 72 \pm 2)then Planck finds 3.6 \pm .25, and 3.046 is excluded at 95% confidence level).

So this story continues, and boils down to the approx 2.5 sigma inconsistency between Planck's value of H0 and the astrophysical estimates. (cf Doddy's comment)

I read that the two anomalies being discussed were hinted at by WMAP and haven't gone away with the (better) PLANCK experiment. I'm not an expert in cosmology or experimental physics, but if the effects haven't gone away, why do you dismiss it? I'm confused.

It's a 2sigma effect, so it's not statistically significant. Plus the fact the quadrupole is aligned with the ecliptic. It's ok to keep it in mind, but I think it was not serious on the part of Planck to push it so much.

There isn't "tension" in the Hubble constant from Planck vs "direct" measurement of supernova distances.

Supernova measurements are just not all that accurate; they're based on a rickety tier of distance assumptions, composition assumptions and even basic Astronomy (not properly accounting for reddening). The error bars are optimistic, at best.

A small error in one or more of the tiers can more than compensate for the difference. Planck's Hubble "constant" value matches the BAO value pretty well; supernova (and other extra galactic distance measurements) just need to improve their game.

I'm trying to grasp the big picture of this amazing big picture (without proper year long studies in cosmology or even particle physics I'm afraid).

Can the power spectrum be somehow read as a signature of a time evolution with larger angles/scales corresponding to early phases of inflation, and small angles to later times in inflation ? Or where the fluctuations at different scales already there at beginning of inflation (and just getting dilated by it) or still taking place simultaneously at all scales during inflation ? At what epoch (if any) are we "dilated away" from a thermal bath at equilibrium ? So many questions...

The answer to all your questions is no :-) I'm better in ridiculing than explaining, but at this link you can find a very pedagogic explanation: http://galileospendulum.org/2013/02/28/c-is-for-cosmic-microwave-background-alphabet-of-cosmology/

Yes, that was a good link as starting point for the lost in the early Universe, and particularly for interpreting the latest CMB power spectrum. I guess I was off by a few dozen orders of magnitude in my futile attempts at mapping cosmology to more mundane physics intuition...

Inevitably I have now other silly questions like "Are the density fluctuations apparent in the CMB the decaying waves of earlier excitations, say a cymbal (without boundaries) struck long ago but still ringing, or travelling waves driven by some more continuous energy releasing process ?" or "How many times a patch that is now, say, seen at 1° angular scale, did significantly oscillate in average density between end of inflation (if any) and recombination ? 10^0 ? 10^40 ?" or "How impossibly huge and cold and exotic a cosmic neutrino background detector would need to be to achieve the same resolution and precision as Planck observatory ?"

But I will keep those questions secret and not dare ridiculing myself again. This is only about the Universe, I shouldn't feel so personally concerned %)Thanks.

The density fluctuations are not decaying, on the contrary. Small primordial fluctuations were seeded by inflation, after which they grow due to the force of gravity. However, the pressure of photons in the plasma provides a counteracting force, hence oscillations. I never studied CMB physics in detail, but if I understand this correctly, the 1st peak at 1 degree corresponds to a half-oscillation, the 2nd to one full oscillations, and so on. I don't know exactly how big a detector you'd need to study the neutrino background, but I guess bigger than the Earth. People have some other ideas how to detect the presence of the neutrino background (e.g. Z bursts), but that won't happen anytime soon.

A bit clumsy sentence but it's correct. ns=1 denotes the scale-invariant spectrum. Inflation predicts ns slightly different than one. Planck definitively proved that ns minus 1 is different from zero, which is what I wrote.

@Rex - Astronomy & Physics still has gravity and dark matter to explore. I think that's where the some of the most exciting research will be in the next 10 - 20 years.

While Planck has done a great job of supporting LCDM, the number of dark matter particles detected is still zero. With the death of sterile neutrinos, the potential zoo is becoming even more exotic.

Most dark matter models do a marginal job of describing the rotational curves in normal galaxies and a miserable job of doing the same with dwarf and low surface brightness galaxies (without invoking uncomfortably unrealistic explanations.)

It'll be great if AMS-02 detected something significant, especially if you can squint at their data and see something similar to the Fermi results (which also requires some squinting...) It could still be an unusual astronomical process (not DM related), but if it is DM, it'll be great news. You would still have to deal with the conflicting mess of results from DAMA/LIBRA, Pamela and ANTARE, but that's another battle.

All DM models really need to work on the galaxy problem; IF DM depended on the data from galactic rotation curves, satellite galaxies, dwarf galaxies and low surface brightness galaxies the theory would have been thrown out years ago, because current DM theories fail miserably at explaining them (DM defenders - please take an honest, dispassionate view at the galaxy data...)

Obviously there IS some form of DM in the Universe. Current theories work great at galaxy cluster and larger scales; they're probably far too simplistic to explain its "fine grained" behavior.

This is why Physics and Astrophysics is still extremely exciting. I don't get the Sturm und Drang over the LHC and Planck results; it's Science - crappy theories get thrown in the trash or modified.

"I don't get the Sturm und Drang over the LHC and Planck results; it's Science - crappy theories get thrown in the trash or modified."

Yeah? So science is all about crappy theories? What about non-crappy theories -- how many of those can draw comfort from the LHC or Planck? Nobody is upset about the death of crappy theories, what we are upset about is the new version of the Second Law -- Boredom either remains constant or increases.

My goodness. Anonymous states that the SN distances are rickety? Please try to read the papers rather than dismiss 23 years since the first modern Hubble diagram from SNe. We are very careful in accounting for the errors in our measurements. Maser distances to NGC 4258, eclipsing binaries, two different types of Cepheid distance measurments, Pop II distance scales all have Hubble constants not consistent at 2 sigma from Planck/WMAP. Remember, the Hubble constant is the *local* expansion of the universe, and the local methods, with their real errors, are direct measurements of H0, whereas Planck is a multicomponent fit to data in the distant Universe. Planck may be right! But unless you have specific objections to the local methods, your simple dismissal is uninformed.

@crappy - If you're a String Theorist or Deeply Truly in love with SUSY, then yeah, I can see how the recent Planck and LHC results are a bit like "The Crying Game" (you know, THAT scene...). This is especially true if you've devoted a career to the above (or worse, just got or are about to get a PhD in the above.)

I'm an Astronomer; if I'd chosen Physics, I would have picked an area where I would have a job for life - something like Plasma Physics - and worked on magnetic confinement fusion.

@ nicholas suntzeff - It may have been a poor choice of words on my part - Other than NGC 4258 (with a 5% error in distance) many other extragalactic distance measurements can have errors of up to 20%; when you consider the statistical and possible systematic errors, it's remarkable that the direct H0 measurements are as close to Planck's - Looking at the published history of H0 measurements over the last 5 to 10 years it averages 67 (pretty good!) with an error of +16/-8

Cepheid distances have improved quite a bit but there's still somewhat of an issue with the values to the LMC Cepheids, for example.

About Résonaances

Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The main goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...